1. Field of the Invention
The present invention relates to CDMA transmission apparatuses used for cellular systems such as digital car telephones and portable telephones.
2. Description of the Related Art
A CDMA (Code Division Multiple Access) system is one of the multiple access system technologies used when a plurality of stations in radio communications such as car-telephones and portable telephones carry out communications on a same frequency band simultaneously. As other technologies, an FDMA (Frequency Division Multiple Access) system, TDMA (Time Division Multiple Access) system, etc. are known. The CDMA system is a system that achieves higher frequency utilization efficiency, capable of accommodating more users than the other technologies.
The CDMA system implements multiple accesses through spread spectrum communications in which an information signal is transmitted with its spectrum spread over a sufficiently wide band relative to the original information bandwidth. One of the CDMA systems is a direct sequence system in which a spreading code is directly carried on the information signal during spreading. In this direct sequence system, signals from multiple mobile stations are multiplexed on a same frequency area and same time zone.
The CDMA system using direct sequence, if a transmitting station which desires communication is located far and another transmitting station which does not desire communication (interfering station) is near, has a so-called xe2x80x9cnear-farxe2x80x9d problem that the reception power of a received signal from the interfering station is greater than that from the transmitting station which desires communication, preventing the stations using only processing gain (spreading gain) from suppressing correlation between spreading codes, which disables communications. For a cellular system using the direct sequence CDMA system, it is therefore indispensable to control transmission power according to the state of each transmission path on the uplinks from mobile stations to a base station.
Furthermore, as a countermeasure for fading which is the cause of deterioration of the line quality in terrestrial mobile communications, a method for compensating variations of instantaneous values of reception power by controlling transmission power is proposed.
Unexamined Japanese Patent Publication No4-502841 includes an example of transmission power control method for a cellular system using a direct sequence CDMA system. FIG. 1 illustrates the configuration to implement the transmission power control method. This cellular system consists of a base station apparatus and mobile station apparatuses and communications are normally carried out between a plurality of mobile station apparatuses and one base station apparatus.
For example, the base station apparatus receives a multiplexed signal from a plurality of mobile stations through antenna 1 and outputs it to analog receiver 2. Analog receiver 2 carries out processing such as amplification and frequency conversion of the received signal and the processed signal is supplied to digital receiver 3 for mobile stations.
Digital receiver 3 separates a signal of a specific mobile station from the multiplexed received signal by performing correlation detection using a spreading code used on the mobile station side and outputs it to baseband processing circuit 4 and reception level detection circuit 6. Baseband processing circuit 4 obtains the reception data from the separated, signal. On the other hand, it outputs the transmission data to the above mobile station to modulator 5 as a transmission signal.
Reception level detection circuit 6 measures the level of the signal received from the above mobile station, generates a power control bit according to the measured level and outputs it to modulator 5. This power control bit is used to control the transmission power of the above mobile station.
Modulator 5 multiplies the transmission signal from baseband processing circuit 4 and power control bit from reception level detection circuit 6 by a spreading code assigned to the above mobile station and outputs the result to adder 7. Adder 7 multiplexes spread signals for a plurality of mobile stations from the modulator. The multiplexed signal is subjected to processing such as conversion to RF frequency and amplification, then transmitted from antenna 1.
The mobile station apparatus receives a signal from the base station through antenna 8 and outputs it to analog receiver 9. Analog receiver 9 carries out processing such as amplification and frequency conversion on the received signal and outputs the processed signal to digital receiver 10. Furthermore, analog receiver 9 is provided with an overall power level measuring circuit for the received signal and the measured power level is input to transmission level control circuit 13.
Digital receiver 10 separates a signal directed to itself from the spread and multiplexed signal through correlative detection and outputs it to baseband processing circuit 11. It also extracts a power control bit from the separated signal and outputs it to transmission level control circuit 13. Baseband processing circuit 11 obtains reception data from the separated signal. At the same time, it outputs the transmission data to the base station as a transmission signal to modulator 12.
Modulator 12 spreads the transmission signal from baseband processing circuit 11 by multiplying it by the assigned spreading code and outputs the result to transmission level control circuit 13. Transmission level control circuit 13 controls the transmission power of the spread signal using the overall power level from analog receiver 9 and the power control bit extracted from the received signal. The output signal of transmission level control circuit 13 is subjected to processing such as conversion to RF frequency and amplification, then transmitted from antenna 8.
The transmission power control method in the cellular system configured as shown above controls the transmission power level using the overall power level measured by analog receiver 2 of the mobile station apparatus. This compensates variations of the central value of the reception level of the base station caused by the varying distance between the mobile station and base station as the mobile station moves. This method is called a xe2x80x9ctransmission power control method based on an open loop.xe2x80x9d
Furthermore, if the level of the signal received from the above mobile station measured by reception level detection circuit 6 of the base station apparatus is lower than a predetermined reference level, the power control bit is determined so that the transmission level of the mobile station may be raised, and to the contrary if it is higher than the reference level, the power control bit is determined so that the level of the mobile station may be lowered, and these power control bits are transmitted to the mobile station.
The mobile station controls the transmission level using the power control bit extracted by digital receiver 10 and carries out compensation for variations of instantaneous values due to fading which is different for the uplink (mobile station xe2x86x92 base station) and downlink (base station xe2x86x92 mobile station). This method is called a xe2x80x9ctransmission power control method based on a closed loop.xe2x80x9d
As shown above, the direct sequence CDMA system uses transmission power control methods based on an open loop and closed loop.
The CDMA system uses codes with high orthogonality as spreading codes to suppress interference between spreading codes. This allows the capacity of the system to be expanded. In Walsh codes and orthogonal Gold codes known as codes with high orthogonality, the number of mutually orthogonal codes is limited to the same number of code length. Therefore, in order to secure the number of spreading codes assigned to the user, a method of combining short codes which have the same cycle as the length of the information symbol and long codes which have longer cycle than that of short codes is adopted.
In this case, such a method is adopted that one long code is assigned to each base station on the downlink, with long codes varying from one base station to another. This allows the orthogonality to be maintained for all users in a same cell. Furthermore, signals from other cells are spread with different long codes and converted to noise, making it possible to suppress interference to a low level. In that case, in order to maintain the orthogonality the number of users is limited to no greater than the code length on the downlink.
On the other hand, on the uplink, since the distance between a mobile station and the base station varies among a plurality of mobile stations and spreading code timing of the received signal in the base station differs from one mobile station to another, it is not possible to maintain the orthogonality of codes. Therefore, the number of users cannot be limited by the code length on the uplink.
Hierarchic type orthogonal codes such as Walsh codes are generated by combining generation matrices hierarchically as shown in FIG. 2, characterized by any two spreading codes in any hierarchy being mutually orthogonal. For example, C10 to C13 are mutually orthogonal, and C20 to C27 are mutually orthogonal. Furthermore, both C20 and C24 consist of elements of C10 and elements with their codes inverted, and thus C20 and C24 are orthogonal to C11 to C13 excluding C10.
In voice communications such as portable telephones, a same communication information rate is used for the uplink and downlink, but when performing multi-med a services in a cellular system such as data communications, the information rate can be asymmetric between the uplink and downlink. Here, communications whose information rate is asymmetric between the uplink and downlink, for example when information is only sent from the mobile station side are called xe2x80x9casymmetric communicationsxe2x80x9d and communications whose information rate is almost identical between the uplink and downlink are called xe2x80x9csymmetric communications.xe2x80x9d
Now suppose a service which carries out information transmission only for the uplink and carries out no transmission for the downlink. Performing closed-loop transmission power control requires securing a spreading code to send a transmission power control bit for the downlink on which no information is transmitted.
In this case, since the spreading code resources of the downlink for closed-loop transmission power control are exhausted, when accommodating services of only downlink signals is attempted a problem occurs that accommodating those services fails because of a shortage of spreading codes even if there is no problem in terms of the system capacity. Furthermore, trying to secure downlink spreading codes assuming that no transmission power control bits are transmitted means not performing transmission power control, which will deteriorate the quality of the uplink and deteriorate the system capacity of the uplink.
It is an objective of the present invention to provide a CDMA communication apparatus in the CDMA cellular system that, even in asymmetric communications with only the uplink, for example, can avoid a shortage of spreading codes on the downlink while carrying out open-loop transmission power control on the uplink.
This objective is achieved during asymmetric communications through the use of a CDMA communication apparatus comprising a frame assembly section for assembling frames with a known reference signal and transmission power bit and a transmission rate control section for setting a lower transmission rate of a transmission signal composed of the known reference signal and transmission power bit above than the transmission rate for symmetric communications.